Coaxial transmission line



July 2o, 194s. l P D, ZOTTU 2,445,380

COAXIAL TRANSMISSION LINE Filed Feb. 16, 1944 3 Sheets-Sheet 1 Z .im

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July 2o, 194s. P. D. zowu 2,445,380

COAXIAL TRANSMISSION LINE Filed Feb. 16, 1944 3 Sheets-Sheet 2` :i l i:a

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COAXIAL TRANSMISS ION LINE Filed Feb. 1e, 1944 s sheets-sheet s l G 57/INVENTOR. BY PAU; D. ZoTTu 'F'CW- Y (/EUWQMQ Aorneg fiar strip or bar ofinsulating -to or carried by a fitting which Patented .July 20t 1948UNITED l srArEs PATENT ortica I Paul to The Gil-dier Corporation,Louisville, Ky.,

tion of Delaware Application February 18,1944, Serial No. 522,647

12 Claiml. (Cl. 174-28) .,by the inner conductor with the yendscontacting vthe inner surface of the outer conductor; and Asmallcylindrical members have sometimes been secured to the outer conductorwith thev ends `abutting against the innerconductor.

The inclusion of a supporting means between vthe inner and outerconductorsalters the distribution of the voltage gradient-and frequently'-causes substantial increases thereof in the iv'icinity oi' theinsulating supporting means. 4The presence of a relatively steep voltagegradient results in ashovers, corona losses and general failure of theline .at voltages much lower than -it is designed and intended towithstand.

These'problems are aggravated When the operatingpotentials are high, asin applications of high frequency to dielectric heating. For example, insuch applications the high frequency voltage may be around 25,000 volts,peak, for the transmission of from one to -three --hundred or morekilowatts to a load.V The frequencies may be of the order of from onemillion to ten million cycles per second, or more, or less.

In carrying out the present invention in one form thereof, the innerconductor oi' a concentric .line is supported by means of a simplerectangumaterial clamped interconnects adjacent sections of the'inne'rconductor and forms an integral part thereof. jThe. supporting bar has aconfiguration such that any change in the normal vpotential gradientexisting 'between `the inner and outer conductors is'minimized. While ofexceedingly simple cons'truction, it has been found that a high voltageconcentric inner con- -ductorsupported in accordance with the present Yinvention operates in a highly satisfactory man- .ner in contrast withAinner conductors for like 2 to be taken in conjunction withtheaccompanying drawings in which;

Fig. 1 is a sectional elevation, taken on the line I-I of Fig. 2. of aconcentric line embodying the present invention;

duties which when supported4 in conventional 'mannerfailed becausepffrequent fiashovers.

-Further objectsand advantages of the present invention will becomeapparent, land will be -pointed out',.in the `following detaileddescription.

Fig. 2 is a sectional view ofthe concentric line of Fig. 1 taken on thelines 2-2;

Fig. 3 is a fractional view taken on the lines 3-3 of Fig. 2;

Fig. 4 is a sectional elevation of a concentric line in which there isdiagrammatically indicated y -an approximately idealcross-section ofl asuitable l supporting bar; *1

Fig. 5 is a sectional elevation of a modified form of the invention;

Fig. 61s a sectional elevation, taken on the lines -I of Fig. 7, of amodification of the invention;l

Fig. 7 is a view taken on the lines I-'I of Fig. 8- in which the outerconductor is rectangular; and

Fig. 8 is a fractional view taken on the lines 8-8 of Fig. '7.

Referring to thedrawings, one form of the invention is shown ascomprising an outer tubular conductor I0 within which there is supportedco axially or concentrically an inner tubular conductor. As shown inFig. 1, the inner conductor is mechanically divided into two parts IIand I2, which are interconnected by a coupling I3. Electrically theinner conductor is continuous. Thus, in assembling the inner conductor,the coupling I3 is moved over the end of the conductor II and solderedthereto, a solder groove I4 being provided for that purpose. Similarlythe conductor I2 is moved into position in the opposite end of thecoupling I3 and soldered thereto with the aid of the groove I5.Preferably shoulders as indicated at I Iiand I I are provided to fix thepositions of the conductors II and I2 within the coupling I3. l

It will be observed the overlying ends I8 and I9 of the coupling I3 arerounded towardsthe conductors II and I2 in avoidance of sharp cornersthe presence of which would result in an increase in the voltagegradient. .Further to reduceA a voltage gradient in the region of thesupporting bar 24, gradient rings 2i and 22 are provided. These may beformed integrally with, or separate from, the coupling I3.

The inner conductor II-I2 I3 is supportedA within the outerlconductor I0by means ofthe insulating bar24 which extends through an opening 25formed centrally and transversely of the coupling I3 by the internalstructure I3a thereof. The supporting member or bar 2| is clampedagainst the upper surface or wall of the opening 2l by means of a plate2l, which may be of brass. The plate 20 is provided with a threadedopening into which there extends the threaded end of a clamping screw21. Its outer end smoothly fits within a. countersunk opening providedin the nttlng II. No sharp edges are present at the surface or around ornear the bar 24.

As shown in Fig. 2, the insulating support 24 extends outwardly of thefitting Il and through rectangular openings 2l and 22 provided in theouter conductor III. These openings are preferably made by cutting theouter conductor along single lines so that the adjacent wail portionsmay be bent outwardly into curved portions as indicated at 28a, 25h, and29a, 29h. If desired, a screw or a pin (such as shown at 58 and B9,Figs. 'I and 8) may extend between these rounded or curved portionswhich engage the support 24. It will thus be seen that the innerconductor is held securely and concentrically in position with referenceto the outer conductor I0. Additional couplings I3 are provided alongthe length of the conductor system of transmission line. Some of thebars may be disposed at a ninety degree angle with respect to othersupporting bars. For example, they may alternate in their direction ofsupport. They function equally well whether the coaxial line behorizontal or vertical or at positions therebetween. For outerconductors of 'rectangular shape, the bars may extend diagonallythereacross.

The positions of the gradient rings 2I and 22 may be varied, and theirheight may be adjusted as the outer conductor is made larger or smaller.In general, high voltage transmission lines of this character arethoroughly tested under simulated or actual operating conditions andilnal adjustments of this character are then made.

Further, in accordance with the invention, a cooling fluid may becirculated through the inner conductor. For this purpose, the couplingI3 is provided with openings 3|, 32, and Il which extend throughinternal structure Ila in directions parallel to`the axis of thecoupling I3 and of the inner conductor. Thus it will be seen that thecoupling I3 electrically interconnects adjacent sections II and I2 ofthe inner conductor and forms a fluid-tight connection therebetween withflow channels for circulation of a cooling fluid, either gas or liquid,through the hollow inner conductor. The couplings further serve tosupport the insulating bars by means of the other opening extendingtransversely of the fluid openings and sealed therefrom. The barsthemselves serve mechanically to interlock the outer and innerconductors while insulating them one from the other with a minimumchange in the voltage gradient therebetween.

Referring to Fig. 4, it will be understood by those skilled in the artthat the electric field surrounding the inner conductor extends radiallytherefrom and at right angles to the magnetic field. For the productionof a minimum change in the voltage gradient each supporting bar may bemade with its boundaries parallel to radial lines taken from the centerof the inner conductor. For example, it may be given a shape as shown atl5. Of course in the region within the inner conductor II the bar Ilneed not decrease in cross section but it is so illustrated the betterto emphasize the overall shape. However, it has been found that thisapproximately ideal shape is not necessary to all applications and thata compromise will be reasonably and commercially satisfactory. Inconsequence, the bar 24 of Figs.

1-3 is made rectangular, as indicated by the broken lines It in Fig. 4.Though there is some deviation from the ideal design outside of theinner conductor, yet the rectangular design is a reasonable compromiseand a satisfactory one. Even though there is interposed in the electricfield an insulating member, there are no regions where the voltagegradient is excessive. On the contrary, it is kept low.

In accordance with a modification of the invention, illustrated in Fig.5, a coupling or tting 31 is arranged for the projection of the ends ofthe inner tubular conductor over extensions 8l and Il. Soldering grooves4l and 4I are provided, as well as shoulders 42 and 4I against which theends of the inner conductor abut. This construction is entirelysatisfactory when the inner diameter of the conductor is of uniformdimension. Ordinarily, however, copper tubing is manufactured with arelatively fixed dimension for its outside diameter. Variations in thewall thickness are reflected by a varying internal diameter. For thisreason, the coupling or fitting Il of Figs. 1 and 2, made of brass orcopper, is preferred for tubing inasmuch as there will always be anassurance of an accurate flt between the outer surface of the tubularconductor and the inner surface of the ends of the coupling Il.

As may be inferred from the foregoing description, the inner conductormay be provided with elongated slots, thereby eliminating the need for aseparate fitting. For example, as shown in Figs. 6 and 7, the innerconductor l0 is provided with axially elongated slots ll and l2 throughwhich an insulating bar Il extends. The central part of the bar 53- hasan opening through which a locking screw I4 extends, the end of thisscrew threadedly engaging a clamping bar Il. The head of the screw I4 iscountersunk into the inner conductor l. and the supporting bar isrigidly held in place by action' of the clamping bar Il and the upperedges of the slots 5I and I2 as viewed in Fig. 7. In this modificationof the invention as well as those above described, the outer conductormay be made in two parts. Both the upper and lower sections Il and i1 ofthe outer conductor may terminate in flanges or outturned ends throughwhich there extends fastening bolts 5I and Il; The outside conductor maybe cylindrical although it is shown as rectangular. As in Fig. 2, theflanges have been bent away to provide room for the insertion of thesupporting bar Il.

In every form of the invention the change in the voltage gradient in andnear the regions of support is minimized. The insulating bar is made ofinsulating material of high dielectric eiliciency. such for example asan insulating material known in the art under a familiar trade name, andcomprising a glass-bonded mica insulating material. Because of therelatively thin or narrow rectangular shape of the bar 24 of Fig. 1 orthe bar II of Fig. 6, the voltage gradient does not rise abruptly in ornear the region of support of the inner conductor. Yet adequate strengthis provided due to the dimension lengthwise of the conductor orcoupling, that is, the greater axial width of each supporting bar. Allair spaces between the inner conductor and the insulating bar have beeneliminated. Small air gaps such as are inherently present withsupporting members of the disc or washer-type have contributed toelectrical failurewith relatively low operating voltages but inaccordance with this invention, such air gaps have been eliminated andthe coaxial transmission line functions satisfactorily at high voltagesbecause of the absence of high voltage gradients on any small or largeportion of the supporting structure.

While particular embodiments of the invention I have been shown, it willbe understood many modifications may be made. It ls therefore intendedby the appended claims to cover any such modifications as fall withinthe spirit and scope of the invention.

What is claimed'is:

1. A transmission line comprising an outer hollow conductor, an innerhollow conductor having a slot elongated lengthwise thereof, means forsupporting said inner conductorin fixed spaced conductor, and means forsecuring in xed position at least one end of said bar to said outerconductor.

2. A coaxial transmission line comprising a hollow outer'conductor and ahollow inner conductor, said inner conductor including a hollow Ycoupling providedwith inwardly extending internal structure having acentrally located opening extending transversely thereof and axiallyelongated with respect thereto, a rectangular supporting bar ofinsulating material extending through said opening in supportingrelation with said coupling and transversely of said inner and outerconductors, said bar having a width axially of said couplingsubstantially greater than its thickness transversely thereof, meanswithin said coupling for rigidly holding said bar and coupling in fixedposition one with the other, and means for attaching the respective endsof said bar to said outer conductor.

3. In a transmission line formed by an inner conductor and an outerconductor radially spaced therefrom, the combination of means forsupporting said inner conductor from said outer conductor comprising acoupling having end portions connected to and forming part of the innerconductor, -said coupling having walls extending inwardly thereof andhaving a rectangular opening extending diametrically across saidcoupling, an insulating member of rectangular cross-section disposedwithin said opening and extending diametrically across said inner andouter conductors with its wider cross-sectional dimension parallel tothe axes of said conductors, said outer conductor having means forsupporting said insulating member at the respective ends thereof, andmeans for clamping said insulating member against said coupling.

y 4. A fluid-cooled transmission line comprising an inner conductorthrough which cooling iiuid may be circulated and an outer conductorconcentric therewith, a hollow coupling forming an integral part of saidinner conductor, said coupling having structure within and extendingtransversely across said coupling, said structure having an -opening forthe passage of fluid from I one side of said structure to the other sidethereof and longitudinally of said inner conductor, said structure alsohaving a separate opening which extends transversely thereof andtransversely of said inner conductor along a diametrical plane Jthereof, an insulating bar extending through said last mentioned openingand outwardly into supinner conductor, said member including internalstructure having at least one Afluid passage extending lengthwisethereof, and a separate opening elongated axially and diametrically ofsaid member and extending transversely therethrough f and isolated fromsaid fluid passage, and means within said separate opening for clampinga supporting member therein.

6. In a conductor system including coaxial inner and outer conductors,the combination of a coupling forming a part of said inner conductor,said coupling having inwardly extending walls having an openingcentrally thereof and extending transversely of the longitudinal axis ofthe inner conductor, an insulating member of relatively smallcross-sectional area disposed within said opening with opposite endsthereof extending to said outer conductor, said outer conductorincluding means for supporting said opposite ends, and means forclamping said insulating member against a Wall of said opening with thecross-sectional area of said member symmetrical withifrespect to adiametral line of said member and of said inner and outer conductors.

7. In a conductor system including coaxial inner and outer conductors,the combination of a coupling forming a part of said inner conductor,said coupling having inwardly extending walls which dene an openingcentrally and transversely thereof. which opening iswider lengthwise ofsaid coupling than is its height, said inwardly extending walls havingat least one additional opening separate and isolated from saidtransverse opening for flow of iiuid longitudinally of said coupling, aninsulating member wider lengthwise of said coupling than its thicknessdisposed within said transverse opening with opposite ends thereofextending to said outer conductor, said outer conductor including meansfor supporting said opposite ends, and means for clamping saidinsulating member against a wall of said transverse opening and in aposition symmetrical with respect to a, common diametral line of saidmember..

of said coupling and of said outer conductor.

8. A coupling member for the inner conductor ofa coaxial transmissionline compri-sing a body portion having an opening extending transverselythereof and diametrically elongated lengthwise of said member,end-portions mechanically and electrically connected to said innerconductor, a rectangular insulating bar extending through said openingand outwardly from said member having a thickness radially of saidmember which is small compared with the width thereof along thelongitudinal axis of said member, a clamping member, and means carriedby said coupling member for holding said clamping member against saidbar to secure it in a fixed position within said opening.

9. A supporting and coupling member for the inner conductor of a coaxialtransmission line comprising a body portion having opposite endportionsconnected to and forming an integral part of said inner conductor, saidmember having an opening extending therethrough transversely of thelongitudinal axis thereof and elongated along said axis, means includinga rectangular insulating member fastened in, and extending outwardlyfrom, said opening for supporting said inner conductor, and gradientrings so disposed on opposite sides of said opening as to reduce thevoltage gradient in the region where said member extends outwardly fromsaid opening.

10. In a coaxial transmission line having an inner tubular conductorformed by a plurality of tubular elements, and an outer tubularconductor, the combination of a coupling member having a body portionand opposite end-portions disposed in overlapping mechanical andelectrical engagement with adjacent tubular elements, said body portionhaving an opening extending transversely thereof and therethrough andelongated along the axis thereof, an insulating bar having a thicknessrelatively small as compared with the diameter of said coupling memberand extending through said opening and outwardly from said couplingmember, a clamping member disposed.

within said opening and engaging one side of said bar, means forpressing said clamping member against said bar to hold it in placeagainst the opposite side of said opening, and means for securing infixed position to said outer conductor at least one end of said bar.

11. A transmission line comprising an outer hollow conductor, an innerhollow conductor having an opening elongated lengthwise thereof andextending diametrically thereof, means for supporting said innerconductor in xed relation within said outer conductor comprising aninsulating bar extending through said opening, means for fastening theends of said bar to said outer conductor, said bar having a rectangularcross-sectional area elongated lengthwise of said inner conductor, andclamping means including a rst member engaging said inner conductor anda second member rotatable to apply pressure to said first member and tosaid bar to clamp it against one side of said opening, said bar in itsclamped position extending symmetrically across said inner conductor. v

12. A high-voltage high-frequency electrical transmission linecomprising an outer hollow conductor, an inner conductorI ofsubstantially smaller diameter than said outer conductor, saidconductors being adapted for operation with l. high voltagetherebetween, the electric field resulting from said high voltageextending in radial lines between said conductors, said inner conductorhaving in diametrically opposite sides thereof aligned slots elongatedlengthwise of said inner conductor, means for supporting said innerconductor in centered relation with said outer conductor in avoidance ofdistorted high potential gradients at the supporting regions in thevicinity of said slots comprising an insuiating bar disposed Within saidslots and extending outwardly into supporting engagement with said outerconductor, said bar having a thickness which is small relative to itswidth lengthwise of said inner conductor, means for holding said bar insaid slots in a symmetrically ilxed position with respect t0 said innerconductor, said thickness oi' said bar between said inner conductor andsaid outer conductor being such that the wide dat surfaces of said bardepart but slightly from said radial lines in avoidance of distortion o!said field and in avoidance of said distorted potential gradients, andsaid width of said bar lengthwise of said inner conductor being adequateto provide the requisite strength for said support of said innerconductor within said outer conductor.

PAUL D. ZOTTU.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Austria June 10, 1932

